Frequently, a so-called LASIK (laser in-situ keratomileusis) technique is used to correct defects of vision of the human eye (for example, short-sightedness or long-sightedness or astigmatism). In this case, a small cover disk (generally referred to as a flap) is first parted off from adjoining corneal tissue, the flap remaining connected to the corneal tissue in a hinge region. This enables the flap to be easily folded away in order to expose the tissue regions of the cornea underneath, and enables the flap to be easily folded back following an ablation of the exposed tissue regions by means of focused UV laser radiation. Removal of material in the ablation procedure causes the surface of the cornea to have an altered shape, after the flap has been folded back, and thus causes the cornea, and consequently the eye system overall, to have a different refractive behaviour. Through appropriate definition of the ablation profile, it is possible to achieve an at least significant reduction in visual defectiveness and, at best, even almost complete correction.
To generate incisions by means of focussed laser radiation in transparent or translucent material (transparent/translucent for the laser radiation), the physical effect of so-called laser-induced optical breakdown is used. The breakdown results in a photodisruption of the irradiated tissue in the region of the focus of the laser radiation. The interaction of the incident laser radiation with the irradiated corneal tissue causes local vaporization of the tissue in the focal point. This may result in the development of gases, which—unless dissipated outwards—collect in internal cavities or are absorbed by adjoining corneal tissue. It has been found that, if gases that develop during production of the flap remain in the cornea in the case of LASIK treatment of the human eye, this may result in problems in the subsequent ablation procedure. In this case, the gases may result in development of a so-called opaque bubble layer (OBL). The development of an OBL may make it more difficult, or even impossible, to precisely track the eye by means of an eye tracker. It is to be noted in this case that laser systems used for the ablation of corneal tissue (as in a LASIK treatment) generally comprise an eye tracker, in order to capture eye movements during the laser treatment and to guide the laser radiation according to the captured movements. Normally, the eye trackers include at least one camera, and appropriate image analysis software for analysing the images recorded by the camera and for capturing changes in the eye position. In this case, characteristic features of the eye (for example, particular points on the iris and/or the center of the pupil and/or the apex of the cornea and/or the limbus) are analysed by the image analysis software. It has been found that gas accumulations (e.g. an OBL) remaining in the cornea, which have occurred during production of the flap, may impede the capturing of such characteristic features of the eye.